Froth flotation is used extensively in industry to separate valuable particles from particles of waste material. In the minerals industry for example, rock containing a valuable component is finely ground and suspended in water, to form a pulp or slurry. Reagents are generally added that attach selectively to the valuable particles making them water repellent or non-wetting (hydrophobic), but leaving the unwanted particles in a wettable (hydrophilic) state. The hydrophobic and hydrophilic particles are referred to as mixed particles. In the minerals industry, the valuable particles are often referred to as “values”, while the waste material is known as “gangue”. Bubbles of air are introduced into the suspension in a vessel or cell. The hydrophobic particles, also referred to as selected particles, attach to the bubbles, and rise with them to the surface of the suspension where a froth layer is formed. The froth flows out of the top of the cell carrying the flotation product. The particles that did not attach to bubbles remain in the liquid and are removed as tailings. Reagents such as frothers may be added, that assist in the creation of a stable froth layer.
The process of adding reagents to the suspension of particles is known as conditioning. Conditioning reagents are usually specific to the particular ore body that is to be treated and the mineral species it contains. The reagents may include a collector, which reacts or adsorbs selectively with the surfaces of the particles to be separated, and a frother, that has the function of stabilizing the bubbles introduced into the system, so that a stable froth layer is formed. Other reagents that may be added, depending on the nature or the ore to be treated, include activators, that assist the collector to adsorb to the particles to be separated, and depressants, that prevent the collector from adsorbing on unwanted minerals.
The formation of a froth layer is an important characteristic of the froth flotation process. In a stable froth layer, froth is discharged over the lip of the flotation cell, being continuously replaced by bubbles with attached particles and entrained particles, from the pulp or slurry in the cell beneath. While moving towards the overflow lip, the froth drains and entrained particles are able to flow back into the pulp, enhancing the purity or grade of the flotation product. The interface between the pulp and the froth is maintained at an appropriate level, so that the froth product can reach the required grade and recovery of particles from the flotation cell.
Machines used in the froth flotation process are known in prior art. A common design consists of an agitator or impeller mounted on a central shaft and immersed in a suitably conditioned pulp in a flotation cell. The rotating impeller creates a turbulent circulating flow within the cell that serves to suspend the particles in the pulp and prevent them from settling in the vessel, to disperse a flow of gas that is introduced into the cell into small bubbles; and to cause the bubbles and particles to come into intimate contact, thereby allowing the hydrophobic particles in the pulp to adhere to the bubbles. The bubbles and attached particles float to the surface of the cell where they form a froth layer that flows over a weir, carrying the flotation product. The impeller customarily is surrounded by a stator that assists in the creation of a highly sheared environment in the vicinity of the impeller, and also prevents the formation of a vortex or whirlpool in the liquid in the cell. Flotation machines of this type, known as mechanical cells, are described in textbooks such as Wills' Mineral Processing Technology, 8th edition, James Finch ed., Elsevier, New York, 2015. Other types of flotation machine, such as column cells, are also described.
It is well known that the recovery of particles in existing flotation devices depends on the size of the particles. For a given floatable species, the recovery of ultrafine particles is very small. With increasing particle size however, the recovery increases until a maximum is reached. With further increases in the particle size, the recovery becomes progressively lower. In base metal flotation, the optimum range for recovery is between 20 and 120 μm in general, although in some cases the upper limit may be slightly increased. Particles above the optimum range are described as coarse particles. For particles of lower density such as coal, the optimum range with current technologies may extend up to 400 μm.
The inability of mechanical cells to recover coarse particles has a detrimental effect on the energy expended in grinding the rock that enters the mill. The grinding energy can be related to the final grind size by an expression known as Bond's Third Law, which can be written:
      Energy    ⁡          (              kW        ⁢                  -                ⁢        hr        ⁢                  /                ⁢        t            )        =      10    ⁢                  W        i            ⁡              (                              1                                          P                80                                              -                      1                                          F                80                                                    )            where Wi is the Bond Work Index, and P80, F80 are the 80% passing sizes (μm) of the grinding mill product and the feed to the mill, respectively. The size of the feed material to the mill is typically 150 mm or greater, so the second term in the brackets is negligibly small compared with the first term. It can be appreciated that if the flotation circuit downstream of the mill could process particles that were much larger than those in current practice, there would be significant savings in grinding energy costs and in the costs of the grinding media such as steel balls and mill linings, the two being proportional. For example, if in a mill where the final grind size is currently 100 μm, the final size could be increased to 400 μm, there would be a 50% reduction in grinding energy and media consumption. Since grinding energy is the largest single energy component in a base-metal concentrator, and a very significant cost in the operation of a complete mine-mill complex, a reduction of energy of this magnitude would lead to massive savings for the whole mining enterprise. Accordingly, there is a long-felt need to be able to float coarse particles, to bring about the savings indicated.
Another long-desired feature of froth flotation technology is the ability to process suspensions with a high fraction of solids. The feed to flotation machines in present technologies is generally in the range 5% for coal to 45% solids in base metal flotation. A system that can accept feeds that are just below the packing limit of the solids, typically up to 75% solids for typical ore suspensions, would be highly beneficial, because of the reduction in the process water demand By increasing the percent solids in the feed, the quantity of recirculated water in the plant will be reduced, as will the demands placed on the downstream thickening and dewatering operations. Furthermore, if the particles are coarser than in current practice, the water lost from the concentrator in the tailings delivered to settling ponds or dams will be considerably reduced. This feature is very important when a concentrator is to be located in a region with limited availability of makeup water.
To mitigate the problem of coarse particle detachment, an invention has recently been disclosed (U.S. Pat. No. 9,085,000), in which flotation is carried out in the relatively calm environment of a fluidised bed. Hydrophobic particles attach to bubbles in the fluidised bed and rise upwardly into a separation zone, in which non-hydrophobic particles detach from the wakes of the rising bubbles and fall back into the fluidised bed, while bubbles with attached hydrophobic particles rise into a froth layer. The froth bubbles flow over a launder lip carrying the hydrophobic particles. Non-hydrophobic particles discharge from the flotation column from the top of the fluidised bed. To maintain the bed in a stable operation, liquid is recycled from a settling zone above the fluidised bed, and returned into the base of the bed.
When a fluidised bed flotation cell is operation, the gas bubbles contact relatively coarse hydrophobic particles in the fluidised bed. The bubble-particle aggregates rise out of the bed and into the separation zone. Many of the relatively finer particles are able to rise upwards to enter the froth zone, discharging over the lip of the containing vessel as a first flotation concentrate. Surprisingly, it has now been observed that not all bubble-particle aggregates have sufficient buoyancy to enter the froth, and they tend to congregate below the froth zone. Given sufficient time, the bubbles coalesce or burst, and the attached particles fall back to the fluidisation zone, or the aggregates may make contact with lightly-loaded bubbles rising in the settling zone, and gain sufficient buoyancy to enter the froth.